An example of a process for the catalytic conversion of a feed mixture which is capable of ignition or explosion is the catalytic partial oxidation of hydrocarbonaceous feedstocks, in particular hydrocarbons. The partial oxidation of paraffinic hydrocarbons is an exothermic reaction represented by the equation:CnH2n+2+n/2O2→n CO+(n+1)H2 
There is literature in abundance on the catalysts and the process conditions for the catalytic partial oxidation of gaseous hydrocarbons, in particular methane. Reference is made, for instance, to EP-A-303 438, U.S. Pat. No. 5,149,464, and International patent application WO 92/11199.
The hydrogen produced by the catalytic partial oxidation process of hydrocarbonaceous feedstocks can suitably be used as feed for a fuel cell. In fuel cells, hydrogen and oxygen are converted into electricity and water. Fuel cell technology is well known in the art.
One of the most challenging applications of fuel cells is in transportation. Transport means, such as automotive vehicles and crafts, powered by fuel cells are under development. The oxygen needed for the fuel cell may be obtained from the ambient air, the hydrogen feed could be obtained from a hydrogen fuel tank but is preferably produced on-board, for example by catalytic reforming of methanol. The on-board production of hydrogen by catalytic reforming of methanol has been proposed, for example by R. A. Lemons, Journal of Power Sources 29 (1990), p 251-264.
Recently, the on-board production of hydrogen by a catalytic partial oxidation process, for example as described in WO99/19249 has been proposed as an alternative for steam reforming of methanol. An important advantage of this catalytic partial oxidation process is its flexibility towards the choice of fuel.
It is important for a power system in transportation applications, that it is able to vary the power output with a factor of at least about 60, preferably of at least 100. Several systems for controlling the power output of fuel cell systems for transportation applications are under development. In U.S. Pat. No. 5,771,476, for example, a system for controlling the power output of a fuel cell is disclosed, wherein the supply of a reactant gas, such as air, to the fuel cell unit is controlled.
Alternatively, in the case of a fuel cell system having an on-board hydrogen-producing unit, the power output may be controlled by regulating the quantity of hydrogen produced. In a catalytic conversion process, the amount of hydrogen produced is directly proportional to the moles of feed mixture that are converted, provided that the process conditions and the composition of the feed mixture are kept constant.
If the supply of feed mixture to a catalyst bed would be varied in the range between the minimum amount desired and up to 60 times that amount, large variations in superficial velocity, residence time, and pressure of the feed mixture would occur. These large variations may result in problems, especially at the lowest feed throughput. Especially in the case of a feed mixture that is capable of explosion and/or ignition, such as in the catalytic conversion of a mixture of hydrocarbons and an oxygen-containing gas, the low superficial velocity of the feed mixture at low throughputs could result in a residence time of the feed mixture upstream of the catalyst which is greater than the auto-ignition delay time, thus causing auto-ignition or particle induced ignition, and in flash-back of flames from the catalyst bed.
It is known to achieve a large variations in output by using a plurality of catalytic reactors, each containing a catalyst provided with a feed supply system, and varying the number of reactors which are turned on, whilst keeping the feed throughput per reactor essentially constant. It will be appreciated that such multi-reactor systems are relatively expensive, since a plurality of feed supply systems, including the flow control equipment, are needed. Moreover, in such a system the output can only be varied stepwise and not continuously. Therefore, there is a need in the art for catalytic reactors that can achieve large variations in output in a single reactor.
It is also important that the reactor comprises a cooling system that is capable of dealing with a varying throughput of hot conversion product, such as hot synthesis gas in the case of a catalytic partial oxidation process. Suitably, the hot conversion product can be cooled by heat exchanging it against air or air/steam mixtures in flexible corrugated metal tubes. Such tubes are known and commercial available, for example from Witzenmann GmbH in Germany. The use of these tubes as heat exchanger is described in EP-A-298 369. Because of their flexibility, the tubes can accommodate thermal stresses due to a varying throughput of hot conversion product.